Inorganic coating compositions formulated using metallic zinc are known to be used as protective coatings on steel substrates to prevent corrosion from forming under different environments. The galvanic protection offered by the metallic zinc has made such coating compositions one of the few that offer good corrosion protection to steel substrates that are used in a marine atmospheric environment. Such coating compositions are formulated to use either a water-based binders or an organic solvent-based binder.
Such coating compositions are known to be used as both preconstruction and full-thickness primers. The main requirements for a preconstruction primer is that it be both weldable and cuttable. A primer is cuttable if it does not reduce the speed at which the primered substrate is cut to a point where the cutting operation becomes uneconomical. A primer is weldable if it allows the primered substrate to be welded without producing a large amount of "blow holes" in the welds, or without causing a large increase in the porosity of the welds. Such blow holes in or increased porosity of the welds is caused by the exposure of the primer to localized high temperatures during welding, causing the primer to vaporize. The creation and presence of such blow holes and increased porosity is not desired because each can result in the failure of the weld, which can result in a structural failure of the welded substrate.
Protective coatings formed from conventional metallic zinc-containing inorganic coating compositions are known to cause blow holes in welds and/or increases in the porosity of welds when exposed to localized high temperature during the welding operation. Such blow holes and increased porosity formed in the welds is caused from the limited ability or complete inability of the coating to recombine itself into a high-melting point product and maintain its integrity when exposed to high temperatures.
Additionally, protective coating formed from water-based metallic zinc inorganic coating compositions are known to use water-soluble binders such as sodium, potassium or lithium silicates. The use of such water-soluble binders is known to produce coating compositions that normally require a considerable amount of time to become water insoluble and, thus provide an effective corrosion barrier in marine atmosphere environments or other high-humidity conditions.
In view of the increasing interest in the environment, it is also becoming increasingly more desirable that coating compositions useful for forming protective coatings have a low or preferably zero volatile organic content and, therefore, be formulated with a water-based binder system.
It is, therefore, desirable to develop a coating composition useful for forming a protective coating for steel substrates that has improved high-temperature resistance, and is capable of retaining its corrosion resistance after exposure to high-temperature conditions of above 750.degree. F. It is desirable that such coating composition also be developed to provide an improved degree of water insolubility and water resistance within a short time after being applied, i.e., provide early water resistance. It is also desired that such coating composition have zero volatile organic content, thereby making its use economically desirable and practical. It is further desirable that such coating composition be formulated so that it be versatile, permitting its use as either a preconstruction or full-thickness primer.